Prior art disclosures of processes for the deposition of ceria particles on a substrate include U.S. Pat. No. 5,063,193 to Bedford et al. which discloses a wash coat of high surface area ceria particles and the preparation thereof in which cerium carbonate is converted to ceria via chemical reactions and calcination; U.S. Pat. No. 6,051,528 to Brezny which discloses a wash coat created by dissolving cerium carbonate to form cerium acetate which is then treated by spray pyrolysis to form a ceria powder; and U.S. Pat. No. 5,989,648 to Phillips which discloses the plasma generation of supported metal catalysts. The referenced patents do not disclose the use of large particle precursors that decompose in a plasma spray to smaller particles that form a catalyst coating on the target substrate.
In a preferred embodiment, the present invention injects a mixture of large size (approximate nominal diameter distribution range >10 micrometers) metal component precursor particles and large size (approximate nominal diameter >10 micrometers) noble metal component particles into a thermal spray to produce a catalyst coating of metal oxide/noble metal having a nominal particle size diameter distribution in the range of <5 microns, particularly <2 or 3 microns, and more preferably <1 micron. (In the ranges stated herein, a particular distribution includes approximately 90% of the particles within the parameter stated.) The invention provides, by a thermal spray process, a porous, fine particle coating of metal oxides and noble metals, having good mechanical adhesion to a substrate surface and stability properties and surface characteristics useful for catalytic reactor applications.
My previous application describes the deposition of metal hydroxides, carbonates and nitrates on a substrate by thermal spray processes. During a line of sight spray process, the precursor materials decompose and oxides are formed that adhere in a mechanically stable manner to the surface of a thin metal substrate. For example, with hydroxide, carbonate and nitrate metal compositions, the reactions respectively occur: a) Me(OH)x→MeOx+H2O; b) Me(CO3)x→MeOx+CO2; and c) Me(NO3)x→MeOx+N2.
The present invention provides an improved catalytic surface and an enhanced method in which the metal oxide precursors, preferably cerium carbonate or cerium hydroxide, become a ceria coating on a metal surface and a noble metal catalyst is deposited on the surface in the same operation. Large particle cerium carbonate is oxidized to cerium oxide, Ce2 (CO3)x. XH2O→Ce2O3+3CO2+XH2O. In the presence of air or oxygen, Ce2O3 is oxidized to CeO2, namely, Ce2O3+½O2=2CeO2. After the thermal spraying, if a substantial quantity of Ce2O3 is present on the substrate surface, an after-treatment may be applied to achieve the correct phase of CeO2. In the process, the large size particles decompose, the water component vaporizes and a ceria and noble metal catalyst coating on the substrate results. More particularly the invention relates to a method of thermally spraying catalyst precursor materials directly onto a substrate surface and the catalyst substrates formed thereby as a result of the pyrolysis reactions occurring in the process. The invention is particularly useful in the deposition of a catalyst material on surfaces of components used in micro-component reaction chamber assemblies.
My referenced parent application for patent describes characteristics and properties of prior art processes and coatings. In the present invention, thermal spraying of large particles produces a coating of small particle size and high surface area on a substrate.